Brent,

I have used some of these Lincoln quotes in other discussions on causes of the war. But, they seem pertinent here, so bear with me.

Executive Mansion,
Washington, August 22, 1862.
Hon. Horace Greeley:
Dear Sir.
I have just read yours of the 19th addressed to myself through the New-York Tribune. If there be in it any statements, or assumptions of fact, which I may know to be erroneous, I do not, now and here, controvert them. If there be in it any inferences which I may believe to be falsely drawn, I do not now and here, argue against them. If there be perceptible in it an impatient and dictatorial tone, I waive it in deference to an old friend, whose heart I have always supposed to be right.
As to the policy I "seem to be pursuing" as you say, I have not meant to leave any one in doubt.
I would save the Union. I would save it the shortest way under the Constitution. The sooner the national authority can be restored; the nearer the Union will be "the Union as it was." If there be those who would not save the Union, unless they could at the same time save slavery, I do not agree with them. If there be those who would not save the Union unless they could at the same time destroy slavery, I do not agree with them. My paramount object in this struggle is to save the Union, and is not either to save or to destroy slavery. If I could save the Union without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do it; and if I could save it by freeing some and leaving others alone, I would also do that. What I do about slavery and the colored race, I do because I believe it helps to save the Union; and what I forbear, I forbear because I do not believe it would help to save the Union. I shall do less whenever I shall believe what I am doing hurts the cause, and I shall do more whenever I shall believe doing more will help the cause. I shall try to correct errors when shown to be errors; and I shall adopt new views so fast as they shall appear to be true views.
I have here stated my purpose according to my view of official duty; and I intend no modification of my oft-expressed personal wish that all men everywhere could be free.
Yours,
A. Lincoln
Source: http://www.abrahamlincolnonline.org/lincoln/speeches/greeley.htm

If it had been about slavery why did Lincoln wait until 1863 to say it was

“So long as I am president, It [the war] shall be carried on for the sole purpose of restoring the union”-Abraham Lincoln Aug 15 1864
Kansas State Journal Sept 15, 1864

https://en.wikipedia.org/wiki/Hampton_Roads_Conference
Lincoln confided to James W. Singleton that his primary concern was the Union. In Singleton's words: "that he never has and never will present any other ultimatum—that he is misunderstood on the subject of slavery—that it shall not stand in the way of peace".[18] HYPERLINK "https://en.wikipedia.org/wiki/Hampton_Roads_Conference"[19] Lincoln's reassurance earned him Singleton's support in the 1864 election.[20]
[20]Cox & Cox, Politics, Principle, and Prejudice (1963)

https://psmag.com/news/remember-that-time-abraham-lincoln-tried-to-get-the-slaves-to-leave-america-55802

Colonization After Emancipation: Lincoln and the Movement for Black Resettlement
Colonization after Emancipation reveals an unexplored chapter of the emancipation story. A valuable contribution to Lincoln studies and Civil War history, this book unearths the facts about an ill-fated project and illuminates just how complex, and even convoluted, Abraham Lincoln’s ideas about the end of slavery really were

The best recorded instance of this was a speech he gave in 1862, when he invited a delegation of black ministers to the White House and basically told them: we’ve undertaken this colonization venture, here’s why I think you should go. He proposes Panama, he alludes to that as the primary colony of the time, but it’s a broader interest as well. He also says: we recognize that slavery is a source of this war that’s being waged, and slavery, by its very presence on this continent, has instilled violence. The way to get around that violence (which is one of the more notorious lines of the speech) is: you need to leave. You will find a peaceful existence abroad, and we will find a peaceful existence here.

The American South did not invent African slavery. It did inherit a British colonial labor system which populated both North and South with African labor; New England’s slave trading and cotton mills helped greatly in perpetuating that labor system as well as Manhattan bankers who extended credit to planters for expansion westward. The great tragedy is that the South was not left to solve the riddle itself, and Northern abolitionists never offered a practical and peaceful solution to what they expressed so much concern over